Refrigerant Pressure–Temperature Calculator

What a PT relationship tells you

While liquid and vapor refrigerant are both present — inside an evaporator or a condenser — pressure and temperature are locked together. Know one and you know the other. That is why a gauge reading can tell you the temperature at which the refrigerant is boiling in the indoor coil or condensing in the outdoor coil.

Once the refrigerant is all vapor (leaving the evaporator) or all liquid (leaving the condenser), the link breaks: its temperature can rise above or fall below the saturation temperature. Those differences are superheat and subcooling.

Dew point, bubble point and glide

A single-component refrigerant such as R-32, R-22 or R-134a boils and condenses at one temperature for a given pressure. A blend boils over a small range: the bubble point is where the first vapor appears in the liquid, and the dew point is where the last liquid disappears. The difference is the temperature glide.

R-410A is a near-azeotropic blend with a glide of only about 0.2 °F, so in practice it behaves almost like a single refrigerant. The calculator still shows both values, because superheat should be measured from the dew point and subcooling from the bubble point.

Gauge pressure and altitude

Refrigerant properties depend on absolute pressure, but a gauge reads pressure above the surrounding air. At sea level the air is about 14.7 psi; at 5,000 feet it is about 12.2 psi. So at altitude the same refrigerant temperature shows a gauge reading roughly 2.5 psi higher. Printed PT charts assume sea level; enter your elevation here to correct for it.

Where the numbers come from

The saturation tables are computed with CoolProp, an open-source property library, and checked against independent published tables before a refrigerant is offered. The Refrigerant PT charts page lists every comparison and explains why R-454B is not offered yet.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

R-410A at 40 °F: 118.4 psig dew point, 118.8 psig bubble point.

Assumptions

• Saturation tables generated with CoolProp 8.0.0 every 1 °F and interpolated linearly; each refrigerant agrees with at least two independent published tables (see the validation report).
• Gauge pressure = absolute pressure − local atmospheric pressure from the standard atmosphere at the site elevation.
• For a blend, the dew point applies to saturated vapor (evaporator outlet, superheat) and the bubble point to saturated liquid (condenser outlet, subcooling).

Limitations

• Gauge accuracy, weather-related changes in barometric pressure and non-condensable gases all affect field readings.
• Use the equipment manufacturer’s charging chart and procedure; this tool gives saturation properties only.

Frequently asked questions

Sources

1. CoolProp — open-source thermophysical property library (version 8.0.0) — CoolProp developers (MIT licence). Equations of state for pure fluids and mixtures, incompressible fluids and brines, and IAPWS-IF97 water/steam; free for commercial use under its licence. Used here to generate the refrigerant saturation tables.
2. Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp — Bell, Wronski, Quoilin & Lemort, Industrial & Engineering Chemistry Research 53(6), 2498–2508 (2014), open access. The citation CoolProp asks users to give; describes the equations of state and their accuracy.
3. Refrigerant PT charts and how they were validated — FreeHVACCalculators.com. Each published refrigerant’s generated table agrees with at least two independent published tables (worst differences and tolerances listed); R-454B is withheld because it does not.
4. NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.

Spotted an error? Report a problem with this calculator — see our corrections policy.